1. Technical Field of the Invention
The present invention relates to a power switching device for a vehicle arranged on a motive power transmission passage from a driving source to wheels.
2. Description of the Prior Art
A power switching mechanism constituting the power switching device for vehicle includes, for example, a two-wheel-drive/four-wheel-drive switching mechanism, a differential lock mechanism, a speed change gear mechanism, or different kinds of clutch mechanisms. Some of the power switching mechanisms have a waiting mechanism which can temporarily accumulate operative power by means of a coli spring.
One type of the power switching mechanism with the coil spring is to utilize compression and tension of the coil spring in an axis direction thereof, and the other type is to utilize torsion or twist of the coil spring around the axis thereof. Japanese Patent publication No. 2011-121458 discloses a differential lock mechanism having the former waiting mechanism, and Japanese Patent publication No. 2000-118257 shows a differential lock mechanism having the latter waiting mechanism.
FIG. 14 shows the latter waiting mechanism, a shared support shaft 201 is supported by a pair of bearing bosses 203 at both ends of the support shaft 201. A drive cylindrical shaft 205 located on an operation side and a driven cylindrical shaft 206 located on an actuation side are rotatably fit on an outer peripheral surface of the support shaft 201 so as to rotate independently each other. A coil spring 210 is disposed on outer peripheral surfaces of the drive and driven cylindrical shafts 205, 206. One end of the coil spring 210 is engaged with an operating arm 205a fixed on the drive cylindrical shaft 205, and another end of the coil spring 210 is engaged with an actuating arm 206a fixed on the driven cylindrical shaft 206.
The operating arm 205a is connected to an electric actuator 211 so as to transmit operative power for switching. The actuating arm 206a has a shift pin 212 which is connected to a differential lock mechanism 213.
When the operating arm 205a is rotated by the electric actuator 211, the actuating arm 206a is rotated through the coil spring 210, so that the differential lock mechanism 213 is switched between locked condition and unlocked condition by means of the shift pin 212. In case that the differential lock mechanism is switched from lock condition to unlock condition, a drive dog teeth and a driven dog teeth (or recess) are changed from disengaged state to engaged state. However, when end faces of both the dog teeth are abutted each other, the coil spring 210 is distorted or twisted around the axis of the coil spring. As a result, the power switching mechanism will come into waiting condition.
In the structure shown in FIG. 14 of the prior art, the drive cylindrical shaft 205 and the driven cylindrical shaft 206 are rotatably supported on the outer peripheral surface of the shared support shaft 201, the coil spring 210 is disposed on the outer peripheral surfaces of the drive and driven cylindrical shafts 205, 206. According to the structure of the prior art, a supporting area of the shared shaft 201 to support the cylindrical shafts 205, 206 is small in area, therefore contact pressure acting on the outer peripheral surface of the shared support shaft 201 becomes large. As a result, operating load becomes large. In case that the operating load becomes large, it is required to provide a big volume coil spring having large spring strength or constant, in order to produce the waiting action.
Further, in case that only the differential lock mechanism is operated, two switching positions shall be sufficient for switching of the power switching device. However, in case that three or more switching positions are required by adding the other switching device, it will be required to increase torsion angle of the coil spring. But, in case that relative torsion angle between the drive cylindrical shaft 205 and the driven cylindrical shaft 206 is increased, the coil spring is contracted or reduced in a radial direction by the torsion of the coil spring. By the contraction of the coil spring in the radial direction, the coil spring is pressed against the outer peripheral surface of the shared support shaft 201. Therefore, it will become difficult to produce smoothly the waiting action.